The present invention relates, in general, to semiconductor devices, and more particularly to superluminescent light emitting devices (LEDs).
Superluminescent LEDs combine high efficiency with a broad spectral bandwidth. These characteristics are important for achieving high performance in applications such as a light source for fiber-optic gyroscopes. Another application for such devices is in a display matrix in which light from several hundred devices is combined to form an image. LEDs which operate in the lasing mode produce an undesirable "speckle" effect in an image due to coherent wave interference. The broad spectral bandwidth of a superluminescent LEDs precludes coherent wave interference effects.
In the past, superluminescent LEDs have been edge emitting devices where the light is emitted parallel to the surface of the semiconductor substrate in which the superluminescent LEDs are fabricated. However, production of light through the top surface of a light emitting device or perpendicular to the semiconductor substrate surface is highly desirable. Superluminescent LEDs which emit light perpendicular to the substrate surface may be integrated with devices having other functions on a single integrated circuit chip. Also, large arrays of similar such devices may be fabricated on a single substrate to provide an economical, high efficiency, planar display. Obviously, edge-emitting superluminescent LEDs used in the past cannot be integrated with other semiconductor devices on the same chip.
Another advantage of superluminescent LEDs which emit light perpendicular to the substrate surface is that they may be assembled easily by direct mounting onto a package surface rather than requiring a difficult "flip" mounting. In addition, testing of the superluminescent LED before separation of the wafer into individual chips is possible. This pre-separation testing avoids the need to mount and package all superluminescent LEDs, of which some may be defective and must then be scrapped. Because packaging costs are a major component of the total superluminescent LED cost, both the ease of assembly and pre-separation testing represent significant manufacturing cost reductions. Furthermore, studies of defect locations on a wafer often provides valuable information which may be used to optimize a wafer manufacturing process. This valuable manufacturing information cannot be obtained if the wafer must be separated into chips before testing.
Superluminescent LEDs operate in a regime between lasing and spontaneous emission. Lasing is undesirable in a superluminescent LED. In the past, there have been difficulties in preventing superluminescent LEDs from lasing within a certain temperature range. In the past, lasing has been prevented by the use of anti-reflective coatings formed on at least one of the facets of an edge-emitting device through which light is emitted. However, driving the superluminescent LEDs to high powers may eventually cause lasing, in spite of the presence of the anti-reflective coating. Because even the best anti-reflective coating will reflect some proportion of the light impinging on it, lasing will eventually occur in the prior art superluminescent LEDs if the power is increased to a high enough level.
Superluminescent LEDs provide high efficiency and freedom from wave interference effects, and superluminescent LEDs which emit light perpendicular to the substrate surface facilitate economical manufacturing, testing, and packaging. There is still a need to fabricate more efficient superluminescent LEDs that do not lase at high power levels and that can be integrated with other devices on the same chip.